Four basic forces govern the flight of an aeroplane:
(1) GRAVITY
Gravity is the natural force that pulls a plane towards the ground.
(2) LIFT
Lift is the force that pushes a plane upwards against the force of gravity. It is created by the movement of a plane's wing through the air.
(3) DRAG
Drag is the natural force of air opposing an aeroplane's forward movement.
(4) THRUST
Thrust is the force that opposes drag and moves a plane forwards. Thrust is created by a plane's propeller or by its jet engines. When a plane's lift equals the force of gravity and its thrust equals the drag, the plane is in level, cruising flight. When any of the four forces changes, the plane begins to climb, turn, or change its direction or position in some other way.

GRAVITY AND LIFT
Gravity tends to keep an aeroplane on the ground or to pull it to the earth when in flight. The force of gravity on the ground equals the weight of the plane on the ground. For a plane to become airborne and to stay in the air, its wing must create a lifting force greater than the downward force of gravity. Lift is created by a change of air pressure around an aeroplane's wing as the plane moves along the ground or through the air.
Early attempts to fly with wings failed because people did not yet understand that it is the curved shape of a bird's wings that creates lift. After this fact was discovered, people began to build aeroplane wings that were slightly curved and so created lift in much the same way as a bird's wings do.
When a plane stands on the ground, the air pressure above and below its wing is the same. As the plane starts to move forward, air begins to flow over and under the wing. The air moving over the curved upper surface flows in a curve. As it does so, its speed increases and its pressure drops. The air moving under the flat bottom of the wing moves in a straight line. Its speed and pressure are not changed by the wing. A high-pressure area always moves towards a low-pressure area, and so the air under the wing tries to move upwards to the air over the wing. But the wing is in the way. Instead of meeting the low-pressure area, the high-pressure area lifts the wing into the air. The faster the aeroplane moves, the greater the lift its wing produces. As an aeroplane increases its speed down the runway before take-off, its wing builds up more and more lift. The air pressure beneath the wing finally becomes greater than the weight of the plane, and so the force of the lift becomes greater than the force of gravity. The plane then takes off.

DRAG AND THRUST
A wing can produce lift only if it is moving forwards through the air. A plane needs engine thrust to create the required forward movement. As thrust increases, it moves a plane forwards faster than before. However, as a plane's speed increases, drag increases also. To oppose drag, the plane needs still more thrust.
In a jet plane, the rapid movement of gases through the jet engine produces thrust. Propellers produce thrust in turboprop planes and planes powered by reciprocating engines. Propeller blades are shaped much like aeroplane wings. As the propeller spins, the air pressure on the front surface of the blades is reduced. The higher pressure on the back of the blades moves towards the lower pressure on the front. As it does so, it pushes against the propeller blades and moves the plane forwards. The faster the jet engine works or the propeller spins, the greater the force of thrust.
To help increase thrust, engineers design aeroplane bodies to be as streamlined as possible. They give them a sleek, trim shape, and they design every part on the outside so that it will knife through the air easily and smoothly.

CHANGING ALTITUDE - An aeroplane cruising in level flight has lift balanced against gravity and thrust balanced against drag. To make the aeroplane descend, the pilot must decrease the power of the engine. The propeller or jet engines slow down, reducing the plane's thrust. The reduction in thrust also reduces lift, and the aeroplane begins to move downwards. At the same time, drag increases its effect, which further slows the aeroplane and adds to the rate at which the plane descends.
In order to climb, the pilot increases the engine power. The propeller or jet engines work faster, creating more thrust. The increased thrust also increases lift, and the plane begins to climb. However, climbing increases drag, and so the plane needs still more lift. To get the added lift, the pilot increases the plane's angle of attack--that is, the angle at which the wing cuts through the air. The pilot uses the controls to make the nose point up slightly so that the wing is at an upward angle to the path of the plane's flight. The flow of air over the upper part of the wing increases in speed, and the air pressure over the wing becomes less than the pressure over the wing in cruising flight. The area of high pressure under the wing moves towards the area of lower pressure over the wing, producing lift. But increasing the angle of attack disturbs the flow of air over the wing, which increases drag. The pilot brings the four forces of flight into balance again by increasing engine power to gain more thrust.

CHANGING DIRECTION - A pilot turns a plane by increasing the lift on one wing or the other. To make a left turn, for example, the pilot operates controls that put the aeroplane into a left bank: that is, the left wing dips lower than the right one. Lift always occurs at a right angle to the surface of the wing. When the wing is not horizontal to the ground, lift takes place at an angle to the ground. As the left wing dips, the lift on the right wing increases, which pulls the plane around the turn. The pilot uses the rudder to keep the aeroplane's nose steady. The rudder is not used to turn the plane. It is the lifting force of the wing, occurring at an angle to the horizon, that makes the aeroplane turn.
When a plane makes a turn, the amount of lift opposing the force of gravity is reduced. As a result, the plane begins to lose altitude. To bring the four forces back into balance, the pilot can do one of two things. The pilot can increase the angle of attack and thus increase the lifting force of the wing. The pilot can increase the engine power to increase thrust. The increased thrust produces greater lift. In making a steep turn, a pilot increases both the angle of attack and the engine power at the same time in order to keep the plane from losing altitude.
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